1. Field of the Invention
The present invention relates to a three-terminal device capable of operating while electrical connection and disconnection between source and drain electrodes are controlled using a gate voltage.
2. Description of the Prior Art
When it comes to a three-terminal device that operates while gaining control of electrical connection and disconnection between source and drain electrodes using a gate voltage, there has a field effect semiconductor transistor so far known in the art. With the field effect semiconductor transistor, the distribution of electrons and holes in the semiconductor is controlled using an electric field induced by a gate voltage so that electrical connection and disconnection between the source and drain electrodes can be controlled. For instance, the operation principles are described at great length in Non-Patent Publication 1.
Among three-terminal devices that control the migration of ions for on-off operation, on the other hand, there has been a device known wherein a positive gate voltage is applied thereby moving metal cations to a source/drain electrode side so that an electrical connection can be made between source and drain electrodes. For its detailed operation principles, see Patent Publication 1. Another example is set forth in Non-Patent Publication 2. In this example, too, a positive gate voltage is applied to supply copper ions from a gate electrode to a source/drain electrode side so that the device can be put on. For its detailed operation principles, see Patent Publication 2.
With the aforesaid prior art field effect semiconductor transistors, any further improvement in performance would not been expectable, in part because there are much more increases in gate leakage currents as device size gets much smaller. For instance, the International Technology Roadmap for Semiconductor has revealed that field effect semiconductor transistors compatible with processing dimensions of less than 11 nanometers are very difficult to develop.
For this reason, three-terminal devices that may replace field effect semiconductor transistors are being enthusiastically developed. In the aforesaid Patent Publication 1 that shows a typical one of them, rather than control of electrons and holes, the distribution of metal cations in an ionic conductor (ion diffusion material) is controlled to achieve certain amount of electron conduction in the ionic conductor. Although this results in source/drain currents depending on gate voltages, it has been found that a problem that sharp on-off operation cannot take place remains unsolved because there is a continuous change in the distribution of metal cations depending on field strength, resulting in the continuous change in the source/drain currents.
According to the teachings of the aforesaid Patent Publication 2, by contrast, the metal ions are reduced on the source/drain electrodes, and metal filaments growing out of the source/drain electrodes couple together, achieving an on-state. In other words, the moment the filaments couple together, there is a decrease in the source/drain resistance, which results in the sharp on-off operation. However, there is another problem that the growth rate of filaments formed by the precipitation of metal ions depends on the value of the applied gate voltage; the gate voltage value at which the on-state is achievable also depends on the operation conditions such as the scan speed of gate voltage. To achieve faster operation, there is a need for using higher gate voltage to boost up the growth rate of filaments. In addition, continued application of the gate voltage causes a filament to be formed even between the gate electrode and the source electrode or the drain electrode, failing to make sure the insulation of the gate electrode.
To provide a solution to those problems, the aforesaid Patent Publication 2 teaches that the spaces between the gate electrode, the source electrode and the drain electrode are controlled such that a filament is formed between the source/drain electrodes before a filament is formed between them and the gate electrode. Even with such an improvement, however, there is still the need for controlling the application time of the gate voltage or the like, because continued application of the gate voltage causes a filament to be formed between the gate electrode and the source/drain electrodes. To put it another way, a problem with Patent Publication 2 is that the device cannot be used as a logic device because of arbitrary application of gate voltage required in the logic operation, so it cannot be a possible replacement of field effect semiconductor transistors.